There have been many investigations in recent years of the effects that small-geometry surface configurations can have on turbulent boundary layers. Particular attention has been paid to the provision of so-called riblet surfaces in which an array of small longitudinal rib-like elements known as riblets extend over the turbulent boundary layer region of a surface in the direction of fluid flow over the surface, to reduce momentum transport or drag. Experimental results indicate that net surface drag reductions of up to about 7% can be achieved.
In a paper "Drag Characteristics of V-Groove and Transverse Curvature Riblets" (presented by M. J. Walsh at the Symposium on Viscous Drag Reduction, Dallas, Tex., Nov. 7-8, 1979) reference is made to an early investigation (1966) by Liu, Kline, and Johnston in which drag reductions of 3-4% were obtained by reducing the turbulent bursting rate, i.e. the rate of break-up of the low speed longitudinal vortices that characteristically form in a turbulent boundary layer flow close to a wall, by employing rectangular fins. Walsh's paper reports the investigation of a number of alternative rib profiles and states he was able to obtain a maximum drag reduction of 7% using V-groove riblets.
In the prior art and later in this specification it is often usual to refer to "streaks" rather than longitudinal vortices, although strictly speaking a streak is merely the visible manifestation of vortices in smoke tunnel experiments, and the present specification will follow this practice.
The drag reduction observed in the experiment referred to above may be associated with the ability of riblets to limit random spanwise movements of the streaks, as has been suggested by a number of sources, e.g. R. E. Falco (AIAA-83-0377, AIAA 21st Aerospace Sciences Meeting, Jan. 10-13 1983, Reno, Nev.). Johansen and Smith (AIAA-85-0547, AIAA-Shear Flow Control Conference, Mar. 12-14, 1985, Boulder, Colo.) have shown that cylindrical riblets of a smaller height than the V-groove riblets with which Walsh obtained his optimum results, may have the effect of anchoring and/or creating sites of low-speed streaks in a limited region above the wall surface, but their experiments also showed drag increases of 3% to 8%.
The results reported from these and other previous investigations all showed that although the effect of riblets may be beneficial it is rather small for practical purposes and that has led to the search for alternative geometries and combinations of techniques offering larger benefits.
In particular, R. F. Blackwelder and J. B. Roon, in "The Effects of Longitudinal Roughness Elements and Local Suction upon the Turbulent Boundary Layer" in "Structure of Turbulence & Drag Reduction" (Ed: A Gyr, Springer 1990) 517)) have attempted to use more widely spaced riblets than those in the earlier experiments referred to, as `streak anchors` in combination with suction through the wall surface in order to improve efficiency. The object was to reduce the number of longitudinal vortices or streaks by placing round wires in the streamwise direction of a wall surface, at such a spanwise spacing that a pair of oppositely rotating streaks could establish themselves over the flat wall surface mainly left clear by the wires. Suction was applied through the surface midway between the wires to the upwardly flowing regions of the streaks in this mid region to weaken the streaks and it was found that the number of streaks were reduced.
The spanwise spacing referred to in the preceding paragraph was 80 wall units, a "wall unit" being the conventional unit in the art for expressing the length values and distributions of the projections. It is a dimensionless unit comprising an actual length value multiplied by a scalar quantity expressing the variation of wall shear stress with fluid density and viscosity.
The 80 wall unit spacing corresponds substantially to the natural spanwise width of each pair of counter-rotating vortices. The wires therefore simply served the function of anchoring the streaks spanwise, being set at the natural spacing of the streaks, and so ensured that the suction apertures were applied to the appropriate regions of the streaks. They had no drag reduction capability themselves. Some advantages have been indicated--a drag reduction of up to 21/2%--but any measure of the net benefit must take account of pumping power required to operate such a system.